1. Field of the Invention
This invention relates to optical coupling technology and more particularly to the use of a movable deflecting element to vary the relative splitting of an optical signal.
2. Description of the Related Art
Embodiments of what are conventionally referred to as optical T-couplers have been previously described in which light guiding glass cores are employed to facilitate coupling between and among several optical fiber channels. For example, U.S. Pat. No. 3,883,217, issued to Love et al., discloses a system employing a glass core light guide to interface opposing bundles of fibers. The light guide length is chosen to insure that light from any one incoming fiber is distributed across the entire aperture of the guide so that the entire receiving bundle is illuminated and data transmitted on all outgoing fibers. This also facilitates the multiplexing of several optical signals onto all outgoing fiber channels simultaneously, which may or may not be desirable, depending on the application. Problems arise in the complexity, component count and manufacturability of the device.
In U.S. Pat. No. 3,870,396, issued to Racki et al., right angle isosceles glass prisms are interposed between terminated bundles of fibers to provide for partial reflection and transmission. Transition light guides, which reduce the cross-sectional area of the device to match the geometry of the fiber bundle are subject to radiative loss.
In U.S. Pat. No. 4,176,908, issued to R. E. Wagner, the concept of a v-shaped groove or notch formed in the light guide is introduced to provide for internal reflection of the beam at substantially 90 degrees to the direction of primary light propagation for the purpose of directing light to a branch port of similar light guiding medium so as to direct a fraction of the total optical power toward the branch, roughly in proportion to the fraction of light guide cross-sectional area occluded by the V-shaped groove.
In the aforementioned disclosures the optical light guides are apparently intended to interface with either bundles of fibers, or appropriate light sources or detectors, such as laser diodes, light emitting diodes or optical photodetectors. The above disclosures are primarily concerned with multi channel distribution of light signals to or between bundles of fibers. This approach has inherent drawbacks due to low power efficiency of distribution into any single fiber channel.
U.S. Pat. No. 4,130,345, issued to O. L. Doellner, discloses the use of single optical fibers in the construction of wedge-shaped building blocks that must be formed, polished, coated with dielectric layers (to facilitate controlled through-transmission and reflection at 90 degrees to the incident direction), and must be aligned and bonded with precision corresponding to the alignment requirements of single or multimode fibers. The use of relatively large diameter glass rods for beam mixing, with respect to the '396 and '908 disclosures mentioned above, is deficient because the reflections from surfaces large in comparison to the wavelength can give rise to time delayed "ghost" signals due to multiple reflections at the light guide end surfaces, but dielectric anti-reflection coatings can significantly reduce this problem, and is, in fact, a common practice. Problems arise concerning the cost of reliably manufacturing such complex coupling devices as described in U.S. Pat. No. 4,130,345. At the time of that invention, multimode fiber was most commonly in use, and the invention would be more tolerant of alignment errors for such fibers than for singlemode fibers. Furthermore, the inventor bases advantages of his invention on incorrectly comparing the outer diameter of typical multimode fibers--125 micrometers (0.005 inches), which is the same for singlemode fibers--to the core diameter of singlemode fibers, typically 10 micrometers or less. The inventor makes no distinction between the fiber diameter and the core diameter. In fact, most standard multimode fibers have core diameters of 50-100 micrometers (0.002-0.004 inches), and are sheathed with an outer glass cladding that is usually approximately the same for both types of fibers to facilitate standard connectorization.
These types of couplers are all variants of a bulk wave beam splitter. Another form of coupler is the fused fiber coupler. In this device, two or more fibers are twisted together and heated while axial tension is applied. As the glass of the fibers softens, the optical cores are brought into close enough proximity that light can transfer from one fiber to the other(s). Such devices are reliable, however, they are usually relatively expensive to manufacture, and function within specifications over a limited range of optical wavelength, because the rate of power transfer depends on the ratio of optical wavelength to physical separation distance between cores of adjacent fibers.
There are no optical lensing elements disclosed in the aforementioned prior art. Clearly it is an advantage to reduce the component complexity in light of prevailing technology. However, the advent of diffractive optics, microlens and gradient index lens technology provides the basis for the development of optical T-couplers, as will be disclosed below, that can easily and efficiently interface single fibers, whether singlemode or multimode, permitting more flexibility in designing optical fiber networks.
All the aforementioned inventions relate to fixed passive couplers, primarily for information distribution. As will be disclosed below, the present invention includes a movable element to permit the variable coupling into respective output ports, such as may be useful for position sensing.